Create internal function to check/add TADA prefixed columns

[hillarymarler]

In several function (see ex below from TADA_ConvertResultUnits), the function begins by checking for TADA prefixed columns and adds them if they do not yet exist. We could create an internal function to do these checks/additions that could be reused as needed instead of duplicated code throughout the package.

[hillarymarler]

@cristinamullin - I could include this with my current work on conversion/harmonization if this is something we are interested in pursuing.